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Potatoes for Processing

Inspection Instructions

August 2015
Non-Discrimination Policy: The U.S. Department of Agriculture (USDA) prohibits discrimination against its customers, employees, 

and applicants for employment on the bases of race, color, national origin, age, disability, sex, gender identity, religion, reprisal, and 

where applicable, political beliefs, marital status, familial or parental status, sexual orientation, or all or part of an individual's income 

is derived from any public assistance program, or protected genetic information in employment or in any program or activity 

conducted or funded by the Department. (Not all prohibited bases will apply to all programs and/or employment activities.) To File 

an Employment Complaint: If you wish to file an employment complaint, you must contact your agency's EEO Counselor (PDF) 

within 45 days of the date of the alleged discriminatory act, event, or in the case of a personnel action. Additional information can be 

found online at http://www.ascr.usda.gov/complaint_filing_file.html. To File a Program Complaint: If you wish to file a Civil Rights 

program complaint of discrimination, complete the USDA Program Discrimination Complaint Form (PDF), found online at 

http://www.ascr.usda.gov/complaint_filing_cust.html, or at any USDA office, or call (866) 632-9992 to request the form. You may 

also write a letter containing all of the information requested in the form. Send your completed complaint form or letter to us by mail 

at U.S. Department of Agriculture, Director, Office of Adjudication, 1400 Independence Avenue, S.W., Washington, D.C. 20250-

9410, by fax (202) 690-7442 or email at program.intake@usda.gov. Persons with Disabilities: Individuals who are deaf, hard of 

hearing or have speech disabilities and you wish to file either an EEO or program complaint please contact USDA through the 

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complaint, please see information above on how to contact us by mail directly or by email. If you require alternative means of 

communication for program information (e.g., Braille, large print, audiotape, etc.) please contact USDA's TARGET Center at (202) 

720-2600 (voice and TDD).
These instructions contain information and guidelines to help personnel of the 

U.S. Department of Agriculture’s (USDA) Specialty Crops Inspection (SCI) Division 

uniformly apply and interpret U.S. grade standards, other similar specifications, and 

special procedures.

These guidelines do not supersede the Federal Food, Drug, and Cosmetic Act or any 

other applicable Federal or State laws or regulations. Compliance with these statutes is 

mandatory. This publication supersedes any previously issued inspection instructions.

The citation of any data, criteria, techniques, illustrations, copyrighted material, or 

pictorial representations accredited to private authorship is used with the permission of 

the individual(s) or source(s) cited. Unless a specific reference is cited, the information 

in this manual has been derived from sources available to the public or from the 

technical knowledge of USDA personnel.

Except for official USDA inspection aids or devices and color guides (or standards) 

produced under license of the Department of Agriculture, the mention of any supplier, 

patented device, product, brand name, or equipment does not imply endorsement by 

USDA over any other similar or equally effective material.

Inspection instructions are issued by USDA after careful consideration of all data and 

views submitted. The Department welcomes suggestions for improving the inspection 

instructions in future revisions. 

Comments may be submitted to: 

Director, Specialty Crops Inspection Division

Fruit and Vegetable Program

USDA, Agricultural Marketing Service

1400 Independence Avenue, SW, STOP 0240 

Washington, DC 20250

These instructions replace the Potatoes for Processing Inspection Instructions dated

May 2015, and include, but not limited to, all previous correspondence, memos, 

inspection instructions, or procedures.
ii

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iii

INSPECTION INSTRUCTIONS FOR POTATOES FOR PROCESSING

SECTION PAGE NO.

GENERAL.......................................................................................................................... 1

INSPECTOR RESPONSIBILITIES ............................................................................................ 1

ADHERENCE TO CONTRACT SPECIFICATIONS ....................................................................... 1

GENERAL COMPUTATIONS.................................................................................................. 2

INSPECTION FACILITIES AND EQUIPMENT ............................................................................. 2

EQUIPMENT ................................................................................................................. 2

SAMPLING......................................................................................................................... 3

SAMPLING METHODS ......................................................................................................... 3

SAMPLING PROCEDURES.................................................................................................... 4

SAMPLING EQUIPMENT....................................................................................................... 5

REMINDERS ABOUT SAMPLING ............................................................................................ 6

HANDLING OF MISSING, DAMAGED, OR INACCURATE SAMPLES .............................................. 6

WATER GAIN..................................................................................................................... 7

DIRT TARE ........................................................................................................................ 8

APPLICATION OF STANDARDS............................................................................................. 9

SUMMARY OF TOLERANCES .............................................................................................. 10

INSPECTION TERMS ......................................................................................................... 10

SIZE DETERMINATION ...................................................................................................... 11

EXTERNAL DEFECTS........................................................................................................ 12

SURFACE AREA ............................................................................................................... 12

CUTTING PROCEDURES FOR EXTERNAL DEFECTS .............................................................. 12

CLASSIFICATION OF DEFECTS (EXTERNAL)........................................................................ 14

BROKEN KNOBS/SECOND GROWTH .................................................................................. 15

FIRMNESS ....................................................................................................................... 15

GROWTH CRACKS ........................................................................................................... 15

NEMATODES ................................................................................................................... 16

SHAPE............................................................................................................................ 16

TRIMMING TO MAKE USABLE PIECES(S) ....................................................................... 16

SIMILAR VARIETAL CHARACTERISTICS ............................................................................... 20

INTERNAL DEFECTS ......................................................................................................... 21

CLASSIFICATION OF DEFECTS (INTERNAL).......................................................................... 21

EQUATION OF AREAS ....................................................................................................... 22

CUTTING PROCEDURES FOR INTERNAL DEFECTS................................................................ 23

DEFECTS NOT ENTIRELY CONFINED TO THE VASCULAR RING ........................................ 23

DEFECTS OCCURRING ENTIRELY WITHIN THE VASCULAR RING ...................................... 24
iv

SAMPLING PROCEDURES FOR INTERNAL DEFECTS ............................................................. 26

SAMPLE SIZE ............................................................................................................. 26

PINK EYE–PINK ROT ....................................................................................................... 27

FREE FROM BASIS........................................................................................................... 27

WASTE BASIS.................................................................................................................. 28

FREE FROM DEFECTS ...................................................................................................... 29

BACTERIAL RING ROT ...................................................................................................... 29

BLACKHEART................................................................................................................... 29

FREEZING / FREEZING INJURY........................................................................................... 30

LATE BLIGHT TUBER ROT ................................................................................................. 30

SOFT ROT / WET BREAKDOWN ......................................................................................... 31

SOUTHERN BACTERIAL WILT (BROWN ROT)....................................................................... 31

WORMS AND INSECTS ...................................................................................................... 32

OPTIONAL TESTS............................................................................................................. 32

SPECIFIC GRAVITY........................................................................................................... 32

SAMPLING FOR SPECIFIC GRAVITY............................................................................... 32

DETERMINING SPECIFIC GRAVITY ................................................................................ 33

EXAMPLE OF SPECIFIC GRAVITY NOTESHEET: .............................................................. 36

TABLE I – CORRECTION FACTORS FOR SPECIFIC GRAVITY OF POTATOES ....................... 37

TABLE II – SPECIFIC GRAVITY CONVERSION CHART ...................................................... 38

GLUCOSE CONTENT......................................................................................................... 39

FRY COLOR..................................................................................................................... 41

BRUISE FREE .................................................................................................................. 42

DETERMINING BRUISE FREE........................................................................................ 42

REMINDERS ABOUT BRUISE FREE ................................................................................ 43
Inspection Instructions for Potatoes for Processing (August 2015)

1

GENERAL

If you encounter problems not covered by these instructions, contact your supervisor. If 

you must take immediate action, use sound judgment and notify your supervisor as 

soon as possible.

Refer to the General Shipping Point Manual, the General Market Manual, and the Fresh 

Fruit and Vegetable Certificate (FV-300) Manual for additional information on subjects

not covered in these instructions.

Any portion of these instructions beginning with §51 and followed with bold print is 

material copied directly from the U.S. Standards for Grades of Potatoes for Processing.

Potatoes for processing are mostly scored on a waste basis rather than on appearance. 

Therefore, the U.S. Standards for Grades of Potatoes for Processing underscore 

describing the amount of waste required to remove the defective portion.

INSPECTOR RESPONSIBILITIES

You are responsible for accurately determining the grade of potatoes for all lots or loads 

specified for inspection.

Contracts between growers and processors usually specify a base price per hundred 

weight (cwt) or ton based on specified grade(s), contract specifications, or a 

combination thereof.

Processors reserve the right by contract to reject all lots or loads of potatoes that do not 

meet the minimum requirements contained in the contract. The Federal-State 

Inspection Service (referred to as the “Inspection Service” in the remainder of these 

instructions) does not have the authority to accept or reject lots or loads of potatoes. 

Accepting or rejecting lots or loads is the processor’s responsibility.

ADHERENCE TO CONTRACT SPECIFICATIONS

Be familiar with the requirements of contract specifications. Contract specifications may 

change during a processing season. When a change to contract specification occurs, 

the processor must notify the appropriate State inspection office, in writing. Do not

grade lots based on the processor’s verbal notification of a contract deviation.

It is the processor’s responsibility to notifying the growers or their representatives of any 

deviation from, or return to, the contract specification.
Inspection Instructions for Potatoes for Processing (August 2015)

2

GENERAL COMPUTATIONS

Unless otherwise specified in a contract, calculate the following on the basis of weight:

• Percentage of dirt and foreign material are determined by dividing the weight of 

dirt and foreign material by the gross weight of the sample less the container 

weight.

• Percentages of all classifications pertaining to potatoes, such as soft rot, 

unusable material, U.S. No. 1, etc. and various categories within these 

classifications, such as U.S. No. 1, 2 inch or 4 ounce minimum, U.S. No. 1, 10-

ounce minimum, etc. are determined by dividing the classifications or categories 

within a classification by the gross weight of clean potatoes.

INSPECTION FACILITIES AND EQUIPMENT

Grading areas and facilities are generally furnished by the processor. It is the 

responsibility of the supervisor, with the cooperation of the processor, to ensure that all 

equipment is in proper working order. To ensure accuracy, have large capacity scales 

inspected and certified by an approved agency. Proper lighting and an adequate water 

supply are critical. Lighting sources which produce natural “daylight” type light are 

preferred at the following sites:

• External Grading Area

• Internal Defect Determination Area

• Glucose Content

• Bruise Free

• Fry Color Determination

EQUIPMENT

Contract specifications and inspection circumstances dictate the equipment needed to 

perform an inspection.

To ensure accuracy, check scales and record weights at the beginning of each shift and 

periodically throughout the day. Record the check weights in a log.
Inspection Instructions for Potatoes for Processing (August 2015)

3

Use a flat and thin bladed knife to determine waste on potatoes, and to cut for internal 

disorders, or to make exploratory cuts. These may include traditional inspection knives, 

paring knives or any similar instrument. Do not use a scoop knife (trim line knife).

Check ring sizers and sizing cards for accuracy with calipers at the beginning of the 

season and periodically during the season.

SAMPLING

A “sample” is a representative part or a single item from a larger whole or group 

presented for inspection or shown as evidence of quality.

The importance of obtaining representative samples cannot be over emphasized. 

Accurate certification is possible only if the samples examined are representative of the 

lot or load.

If sampling is not under the direct supervision of the Inspection Service, it is the 

supervisor’s responsibility to report this on the notesheets or certificates.

SAMPLING METHODS

Sampling may be drawn either by hand or mechanical techniques:

Hand Sampling: An official sample obtained by manually passing a tray through the 

flow of potatoes by a licensed sample puller.

Mechanical Sampling: An official sample obtained mechanically by a licensed sample 

puller. The mechanical sampler will remove approximately the same amount as the 

hand tray.

Note: The Inspection Service must approve all mechanical samplers used for official 

sampling.

Three types of approved sampling techniques are currently used. These are: 

1. Federal-State Sampling: The sampler is either a licensed employee or other

authorized personnel of the Inspection Service. The Inspection Service assumes 

full responsibility for employees in this position, ensuring that proper sampling 

methods and procedures are followed.

2. Supervised Sampling: The sampler is an employee of the company and has 

received documented training by the Inspection Service in the correct sampling 

policy, procedures and ethics. Although there may be company personnel 

overseeing the sampler(s), the responsibility of proper sampling methods, 
Inspection Instructions for Potatoes for Processing (August 2015)

4

procedures, sample identity and security remain with the Inspection Service.

Document and schedule visits by Inspection Service supervisory staff to ensure 

good supervision. Notesheets and certificates or computer printouts from this 

type of sampling must indicate that “Supervised Sampling” was used. If proper 

sampling methods and procedures were not followed, designate the sample as a 

“submitted” sample.

3. Submitted Samples: Samples are drawn, identified and controlled by the 

processor and/or grower, and submitted to the Inspection Service for grading. 

Notesheets and certificates or computer printouts resulting from this type of 

sampling must show “Submitted Samples.”

Certification is limited to the potatoes within the sample only. The following statements 

may be used:

• “This certificate restricted to submitted samples which the company states covers 

750-100 pound sacks.”

• “This certificate restricted to samples submitted by company personnel who state 

covers the delivery of 40,000 pounds of potatoes.”

• “This certificate restricted to sample weight only.”

The supervisor is ultimately responsible for the integrity of the sample.

SAMPLING PROCEDURES

The sample must be representative of the quality, condition, size, dirt and foreign 

material of the whole load/lot. Unless specified by request or contract, do not have the 

sample represent less than 1/2 of 1 percent of the total gross weight. Larger sampling 

rates are acceptable.

Sampling procedures will vary depending upon the processing areas and physical setup 

of the grading line. Typically the sample is drawn and divided or split mechanically, only 

a portion of the sample is graded, with the remaining portion being used for additional 

tests such as bruise free or specific gravity determinations. Other times the entire 

sample is graded. The graded portion of the split sample is generally half, but may be 

specified by request or contract.

Larger sample size and the frequency of samples will reduce the chances of inaccurate 

results. For example, twenty properly drawn 25 pound samples should be more 

representative than ten properly drawn 50 pound samples. Increasing the frequency of 

samples improves the chances of sampling all parts of a load and also reduces the 

possibility of inaccurate results.
Inspection Instructions for Potatoes for Processing (August 2015)

5

It is the responsibility of the Inspection Service to outline sampling procedures. Due to 

varied and assorted types of equipment used, it may be necessary to reassess the 

circumstances and apply sampling procedures more appropriate to the particular 

situation.

States that perform sampling use sample trays that are similar, but may vary in size and 

shape. Sampling is done by holding the tray at the end of a bulk truck’s unloading 

apparatus as the potatoes fall into conveyors that will in turn deposit the potatoes in a 

pile or heap.

Dirt, stones, foreign material, clods as well as adhering material, deposited in bulk 

trucks by mechanical harvesters may be a problem. When potatoes are unloaded from 

the truck, loose dirt tends to localize in the area of the unloading belt or chain. 

Processors emphasize the accurate determination of the percentage of dirt, or material

other than potatoes present in a lot of potatoes. Some contracts specify the type of 

unloading machinery allowed on trucks.

To properly sample, hold the tray in a fixed position. When the unloading belt or chain 

is wider than the sample tray, use one of the following methods for drawing samples:

• Draw sample from alternate sides of the unloading belt; or,

• “Pass through” the sample tray from one side of the belt to the other. This may 

be strenuous, but should not in any way sacrifice the accuracy of the samples.

After sample is drawn, carefully deposit it in a sample bin(s), burlap sack(s) or film 

bag(s). This could include that day’s run for individual growers. A single day’s 

harvesting from the same field for the individual grower may constitute a single lot under 

that grower’s purchase order number.

SAMPLING EQUIPMENT

Lidded Bin System: This system is by far the most secure and easiest to control. 

Each bin holds approximately 1,000 pounds of potatoes. After the bins are filled, they 

are sealed; sample slips bearing pertinent information are either inserted in the bin or 

attached to the outside of the bin. It has been noted that double-lidded bins are the 

most efficient.

Another advantage of the bin system is that the bins at the grading station are usually 

individually weighed and handled by machinery and dumped mechanically. This 

method helps to stabilize the accuracy of the dirt tare factor and helps to eliminate 

bruises caused by handling.
Inspection Instructions for Potatoes for Processing (August 2015)

6

Sack System: This system utilizes burlap sacks; film lined burlap sacks, or even 

heavy-duty film bags. Potatoes that are part of a sample are emptied into the sack or 

bag, usually containing up to 80 pounds. The biggest disadvantage with the sack 

system is the possibility of losing dirt, increased bruising and the additional labor 

needed for handling.

REMINDERS ABOUT SAMPLING

• Sample representatively for accurate certification.

• Supervisors are responsible for sample selection and must correct imbalances

when found in the process.

• Check sample bins or bags for any apparent damage.

• Eliminate all chance for possible sample tampering.

• No unauthorized people in the sampling area unless accompanied by the 

sampling supervisor or sampler.

• Do not tolerate interference from any party. Report such incidents to the 

supervisor immediately.

HANDLING OF MISSING, DAMAGED, OR INACCURATE SAMPLES

The following instructions cover the handling of missing, damaged or inaccurate 

samples and refer to samples officially drawn or supervised by the Inspection Service. 

In all cases the responsible supervisor or the inspector in charge must carefully assess 

the situation before applying corrective measures.

Scale Ticket(s) Received on Unsampled Loads

• Group the unsampled ticket number(s) with their corresponding weight(s) on the 

notesheet. Adjacent to each unsampled ticket number(s) show that no sample(s) 

were taken from the load. Subtract the weight of the load(s) with missing 

samples from the total weight of the lot.

• Notify the applicant of the facts concerning unsampled load(s). If the applicant 

requests that unsampled load(s) and their scale ticket number(s) be included on 

the certificate or produce receipt, obtain a written statement from the applicant. 

Use the Request for Inspection Form or provide a blanket request that covers the 

entire season sampling period.
Inspection Instructions for Potatoes for Processing (August 2015)

7

Damaged Samples

• On the notesheet, group the damaged sample ticket numbers with their 

corresponding weight. Adjacent each ticket number; indicate the samples were 

damaged and not suitable for inspection. Subtract the weight of the 

corresponding ticket number(s) with damaged sample(s) from the total weight of 

the lot. If a portion of the lot was not sampled, or if there were samples that were 

damaged and not included in the inspection process, report the fact(s) on the 

certificate.

• Notify the applicant that the sample(s) were damaged and could not be used for 

inspection results. If the applicant requests that the scale ticket number(s) of the 

corresponding damaged sample(s) be included on the certificate or produce 

receipt, request a statement from the applicant in writing. Use the “Request for 

Inspection Form” or provide a blanket request that covers the entire season 

sampling period.

• Do not score damage that is clearly from grading machinery.

Obvious Errors

If an obvious error has occurred, do not use previous or post inspection results to 

average or aid in determining what corrections are necessary. If essential information is 

not available to make necessary corrections, write a remarks statement on the report or 

certificate that clearly indicates which attributes of the lot are not certified.

WATER GAIN

The “water gain” (also called “water adjustment”) is the amount of water a lot of 

potatoes will pick up in the weight during the washing process.

Some contracts will specify the percentage of water gain. The computation for water 

gain is shown only on the inspection notesheet.

Unless otherwise specified or agreed to by buyer and seller, compute the water gain at 

1%. The percentage used for water gain must be common knowledge to all parties 

when adjusting the net weight of the sample.
Inspection Instructions for Potatoes for Processing (August 2015)

8

DIRT TARE

Deductions for dirt, rocks, vines and foreign material are determined by the following

process:

• Weigh samples in containers (gross) and dump.

• Weigh empty containers (tare). Subtract tare from gross to equal total potato and 

dirt weight.

• Wash, grade and re-weigh potatoes to obtain potato and water weight.

• Subtract water gain equals total potato weight.

• Subtract the total potato weight from the total potato and dirt weight. 

• The difference equals dirt tare.

Another process for determining dirt tare is:

• Weigh samples in containers (gross) and dump.

• Weigh empty containers (tare).

• Wash and re-weigh potatoes before dividing the sample.

• Water gain is subtracted from the total potato weight.

• Subtract total potato weight from the total sample weight, less container weight.

• The difference equals dirt tare.

Either process achieves dirt tare. The two processes are summarized as follows:

 Sample Weighed Washed Potatoes Potatoes and Dirt

– Empty Containers – Water Gain – Potatoes

= Potatoes and dirt = Potatoes = Dirt

Or

 Gross Weight Gross Contents

– Container Weight – Potatoes (corrected for water gain)

= Gross Contents = Dirt
Inspection Instructions for Potatoes for Processing (August 2015)

9

APPLICATION OF STANDARDS

In any lot which fails to meet the requirement of the respective grades or size 

categories, the general application of tolerances does not apply when determining the 

percentages of potatoes. However, for lots which are graded or pre-sorted for size or 

quality and offered for inspection and are required meeting one of the grades, the 

following tolerances by weight are provided.

§51.3414 Application of standards...

(a) For defects:

(1) U.S. No. 1 Processing: 10 percent for potatoes which fail to meet the 

requirements of this grade including not more than one-half of this 

tolerance, or 5 percent, for serious damage by any means, including 

therein not more than 2 percent for potatoes which are frozen or 

affected by soft rot or wet breakdown.

(2) U.S. No. 2 Processing: 10 percent for potatoes which fail to 

meet the requirements of this grade including not more than 

one-half of this tolerance, or 5 percent, for potatoes which are 

seriously damaged by internal defects occurring entirely 

within the vascular ring, including therein not more than 2 

percent for potatoes which are frozen or affected by soft rot or 

wet breakdown.

(b) For loose sprouts, dirt and foreign material: 2 percent.

(c) For off-size:

(1) Undersize: 3 percent when the minimum size specified is less 

than 2-1/4 inches in diameter or less than 5 ounces in weight; 

and 5 percent when the minimum size specified is 2-1/4 inches 

or more in diameter or 5 ounces or more in weight.

(2) Oversize: 10 percent.
Inspection Instructions for Potatoes for Processing (August 2015)

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SUMMARY OF TOLERANCES

U.S. No. 1 Processing

• 10% total defects, including

• 5% serious damage, including

• 2% frozen, soft rot, or wet 

breakdown.

U.S. No. 2 Processing

• 10% total defects, including

• 5% serious damage by internal 

defects entirely within the vascular 

ring, including

• 2% frozen, soft rot, or wet 

breakdown.

U.S. No. 1 Processing

U.S. No. 2 Processing

• 2% loose sprouts, dirt, and foreign 

material.

• 3% undersize (except 5% 

undersize for 2-1/4 inch diameter 

and larger, or 5 ounce minimum 

and larger).

• 10% oversize.

INSPECTION TERMS

Use the following definitions in the grading of potatoes for processing:

Damage: Any defect or combination of defects other than those listed in the 

“Classification of Defects” Tables (External / Internal), which cannot be removed without 

a loss of more than 5% of the total weight of the potato.

Exploratory Cuts: The cut made to determine the presence of hidden defective 

material. This could be the surface appearance of cuts, crevices, growth cracks, 

lenticels, folded ends, superficial bruises or any similar factor. Do not consider the flesh 

exposed in the process of making an exploratory cut when determining if more than 

50% of the tuber’s surface is exposed.

Foreign Material: Applies to seed pieces and material other than potatoes.

Serious Damage: Any defect or combination of defects, other than those listed in the 

“Classification of Defects” Tables (External / Internal), which cannot be removed without 

a loss of more than 10% of the total weight of the potato.
Inspection Instructions for Potatoes for Processing (August 2015)

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Unusable Material: Consists of defective portions of potatoes, and potatoes which are 

frozen, affected by freezing injury, soft rot, wet breakdown, Blackheart, Late Blight 

Tuber Rot, Southern Bacterial Wilt, Bacterial Ring Rot, or which are seriously damaged 

by internal defects or contain insects, worms or larvae.

Usable Piece: The portion of the potato remaining after trimming or as it occurs in the 

sample. Provided, that it meets the following requirements:

• Does not have any unusable material;

• Unless otherwise specified, weighs at least 4 ounces; and,

• At least 50% of peel remains after trimming.

As it occurs in the sample means the individual potato or piece of potato found in the 

sample at time of inspection.

The possible determination of a usable piece depends on one of the following 

conditions:

• Defective material by weight exceeds 10%;

• Aggregate (external) surface area exposed in specific defects exceed that 

allowed in U.S. No. 2 Processing; and,

• Area affected (internal) is greater than that allowed in U.S. No. 2 Processing, 

except in the case of those internal defects which are considered as unusable 

material.

The area exposed either mechanically prior to grading, or the area exposed in removing 

defective material, when in combination cannot expose more than 50% of the surface 

area.

Waste: The defective portion of the potato that remains after the potato has been 

peeled in the normal preparation for processing.

SIZE DETERMINATION

Determine size only in terms of diameter or weight.

Diameter: The greatest dimension in terms of inches or fractions of an inch measured

at right angles to the longitudinal axis, without regard to the position of the stem end.
Inspection Instructions for Potatoes for Processing (August 2015)

12

Weight: The minimum or maximum weight of a potato measured in terms of whole 

ounces.

When a maximum size is designated, the potato is not considered being oversize until it 

weighs to the next highest ounce.

A potato is not considered as meeting any size or any incentive size, such as 10 

ounces, until it is graded or trimmed (or specified otherwise by contract). When 

determining weight of a usable piece, only the usable piece is weighed.

Unless otherwise specified, the minimum size for U.S. No. 1 Processing is 2 inches or 4 

ounces and for U.S. No. 2 Processing 1-1/2 inches or 4 ounces in weight.

EXTERNAL DEFECTS

§51.3415 Definitions.

“External Defects” are defects which can be detected externally. Cutting may be 

required to determine the extent of the injury.

SURFACE AREA

Surface Area: When used in conjunction with the Classification of Defects Tables 

(External / Internal), means that the aggregate area of the defect will not exceed the 

percentage of surface specified.

Judge defects permitting a percentage of surface area or a percentage of waste on the 

following basis:

• Score against the respective grade when exceeding the aggregate surface area 

permitted.

• Score against the respective grade when the allowable aggregate surface area is 

not exceeded for the applicable grade, but has waste material exceeding 5% or 

10% respectively.

CUTTING PROCEDURES FOR EXTERNAL DEFECTS

Waste Basis: Potatoes having more than 10% waste by weight or having defects 

exceeding a specified area will not meet U.S. No. 2 Processing requirements. Unless 

prohibited by contract, trim away all defective material to determine whether or not a 

“usable piece” remains.
Inspection Instructions for Potatoes for Processing (August 2015)

13

Method of Trimming: Remove defective material by flat, curved or scoop cuts only. 

Use whichever removes the least amount of non-defective material. Do not use a veecut or boring cut. When removing defective material, include only an acceptable 

amount of uninjured tissue, such as in the normal preparation for processing.

Remove defective areas with care on an individual potato. When several defects are 

located on the individual potato, make every effort to remove defective portions with one 

smooth cut. When judging cuts as they occur in the sample, make an exploratory cut to 

determine if defective material is masked or disguised under the cut surface. Also 

examine the discoloration, the area covered by dirt, or healed scar tissues of the cut 

surface.

Peelable and Non-peelable Areas: When applying a waste basis to a combination of 

peelable and non-peelable areas, only consider the non-peelable areas as defective 

material.
Inspection Instructions for Potatoes for Processing (August 2015)

14

CLASSIFICATION OF DEFECTS (EXTERNAL)

Defects Maximum Allowed for 

U.S. No. 1 Processing

Maximum Allowed 

for U.S. No. 2 

Processing

Air cracks 5% waste 10% waste

Blackleg 5% waste 10% waste

Bruises 5% waste 10% waste

Cuts Smooth, not more than 

10% of surface

Smooth, not more 

than 1/3 of surface

Enlarged lenticels, discolored or 

sunken 5% waste 10% waste

Flea beetle 5% waste 10% waste

Folded end 5% waste 10% waste

Fusarium tuber rot 5% waste 10% waste

Grass, wireworm 5% waste 10% waste

Greening, light 5% waste 10% waste

Growth cracks USDA Visual Aid1 10% waste

Grub 5% waste 10% waste

Ingrown sprouts 5% waste 10% waste

Nematodes 5% waste 10% waste

Rhizoctonia, solid or 

thick mounded

Not more than 10% of the 

surface.

Not more than 25% 

of the surface

Rodent, bird damage 5% waste 10% waste

Scab, pitted 5% waste 10% waste

Scab, russet 25% of surface or 

5% waste 

50% of surface or

10% waste

Scab, surface 25% of surface or

5% waste

50% of surface or

10% waste

Scab, surface

(elephant hide)

10% of surface or

5% waste

25% of surface or

10% waste

Second growth 5% waste 10% waste

Scaling, surface cracks – when 

cracked, thick and has depth

10% of surface or

5% waste

25% of surface or

10% waste

Sunburn 5% waste 10% waste

Sunken discolored areas 5% waste 10% waste

 1 POT-L-1, USDA Visual Aid maximum allowed for U.S. No. 1
Inspection Instructions for Potatoes for Processing (August 2015)

15

BROKEN KNOBS/SECOND GROWTH

Potatoes that do not meet the requirements of U.S. No. 1 Processing due to one or 

more broken knobs (as it may occur in the sample) may be classed as U.S. No. 2 

Processing on the basis of cuts (not more than 1/3 of the surface exposed and not 

jagged or rough). However, if the same potato has an additional defect(s), trim all 

defective material. After trimming, the potato would be classed as a usable piece, 

provided it meets the requirements as defined in the U.S. Standards.

FIRMNESS

Both grades require potatoes to be “moderately firm.”

§51.3415 Definitions...“Moderately firm” means that the potato is not 

seriously shriveled or flabby.

Scoring Guide

Serious damage: Report potatoes that are seriously shriveled or flabby as “Not 

moderately firm” and score against the serious damage tolerance for all grades.

GROWTH CRACKS

Scoring Guide

Damage: When a growth crack(s) affects more than 1/2 the length of the potato in the 

aggregate on round varieties (see USDA Visual Aid POT-L-1, page 25, photo 118), or 

more than 1/3 length in the aggregate on long varieties, or the depth is greater than 

outlined in the chart below.

DEPTH ALLOWED FOR GROWTH CRACKS 

Diameter Weight U.S. No. 1 (Depth)

Potato is: Potato is: Not more than:

Less than 2 inches Less than 4 ounces 1/8 inch

2 to 2-1/2 inches 4 to 6 ounces 1/4 inch

More than

2-1/2 to 3 inches

More than

6 to 8 ounces 3/8 inch

More than 3 inches More than 8 ounces 1/2 inch

Serious damage: When growth crack(s) affects more than 10% waste, or appearance 

is worse than USDA Visual Aid POT-L-1, page 25, photos 119 and 120.
Inspection Instructions for Potatoes for Processing (August 2015)

16

NEMATODES

Scoring Guide

Damage: When causing more than 5% waste.

Serious damage: When causing more than 10% waste. When seriously damaged, do 

not attempt to form a usable piece.

NOTE: If nematodes are found in potatoes with little external indications, determine 

“damage” and “serious damage” on a composite sample basis. Follow the same 

sampling procedure used in determining percentages for internal defects (see Internal 

Defects section).

SHAPE

The U.S. No. 1 Processing grade requires potatoes to be “fairly well shaped.” The U.S. 

No. 2 Processing grade requires potatoes to be "not seriously misshapen."

§51.3415 Definitions.

“Fairly well shaped” means that the potato is not materially pointed, dumbbellshaped, or otherwise deformed.

“Not seriously misshapen” means that the potato is not seriously deformed and 

causing more than 10% waste when determining the non-peelable area directly 

due to shape.

Scoring Guide

Damage: When not meeting the requirements of “Fairly well shaped” report as “Not 

fairly well shaped.”

Serious damage: When not meeting the requirements of “Not Seriously Misshapen” 

report as “Seriously misshapen.”

Visual Aids: Refer to the Official Visual Aids for Potatoes, USDA Visual Aid POT.-L-1, 

pages 3 to 4, photos 14 to 22.

TRIMMING TO MAKE USABLE PIECES(S)

When a tuber is seriously misshapen (e.g., causing an excess of 10% waste of nonpeelable area), the potatoes may be trimmed to make a usable piece provided that the 

following three criteria are met.
Inspection Instructions for Potatoes for Processing (August 2015)

17

• There is no unusable material, such as defective portions of potatoes, and 

potatoes which are frozen, affected by freezing injury, soft rot, wet breakdown, 

insects, worms, larvae, Blackheart, Late Blight Tuber Rot, Southern Bacterial 

Wilt, Bacterial Ring Rot, or which are seriously damaged by internal defects.

• Must have at least 50% of the peel remaining after trimming.

• Weigh at least 4 ounces, unless otherwise specified.

NOTE: When trimming potatoes, always cut to maximize value and use.

See following examples for trimming potatoes to make a usable piece(s).
Inspection Instructions for Potatoes for Processing (August 2015)

18

Example 1: After determining that there is more than 10% waste due to non-peelable 

area, follow the three criteria for usable piece and trim the potato creating as little waste 

as possible.

Example 2: After determining that there is more than 10% waste due to non-peelable 

area, determine the best cut to reduce waste. In this case, cut the potato in half to 

make two usable pieces (if both are 4 oz. and larger and at least 50% of peel remains 

after trimming). If not meeting criteria, or if a larger piece is desired, cut off one of the 

lobes to make one usable piece.
Inspection Instructions for Potatoes for Processing (August 2015)

19

Example 3: If more than 10% waste due to non-peelable area, then trim as follows:

Example 4: If more than 10% waste due to non-peelable area, then trim as follows:
Inspection Instructions for Potatoes for Processing (August 2015)

20

SIMILAR VARIETAL CHARACTERISTICS

Both grades require potatoes to be of “similar varietal characteristics.”

§51.3415 Definitions...“Similar varietal characteristics” means that the 

potatoes in any lot have the same general shape, color, and character of 

skin and color of flesh.

Scoring Guide

Report potatoes that are not of similar varietal characteristics as “Dissimilar varietal 

characteristics” and score against the total tolerance of the grade being applied.
Inspection Instructions for Potatoes for Processing (August 2015)

21

INTERNAL DEFECTS

§51.3415 Definitions...“Internal Defects” are defects which cannot be detected 

without cutting the potato.

CLASSIFICATION OF DEFECTS (INTERNAL)

Defects Maximum allowed for U.S. 

No. 1 Processing

Maximum allowed for U.S. 

No. 2 Processing

Occurring outside of, or not entirely confined to the vascular ring

Internal Black Spot

When spots are darker than 

the official color chip (POTCC-2) after removing 5% of 

total weight of the potato.

When spots are darker than 

the official color chip (POTCC-2) after removing 10% 

of total weight of the potato.

Internal Discoloration, 

Vascular Browning, 

Fusarium Wilt,

Net Necrosis,

Other Necrosis,

Stem End Browning.

5% waste. 10% waste.

Occurring entirely within the vascular ring

Hollow Heart or Hollow 

Heart with Discoloration.

Area affected not to exceed 

that of a circle 7/8 inch in 

diameter in a 10 ounce 

potato.1

Area affected not to exceed 

that of a circle

1-1/4 inches in diameter in 

a 10 ounce potato.1

Light Brown 

Discoloration (Brown 

Center).

Area affected not to exceed 

that of a circle 7/8 inch in 

diameter in a 10 ounce 

potato.1

Area affected not to exceed 

that of a circle

1-1/4 inches in diameter in 

a 10 ounce potato.1

Internal Brown Spot and 

similar discoloration

(Heat Necrosis).

Not more than the 

equivalent of 3 scattered 

spots 1/8 inch in diameter in 

a potato 6 ounces in weight 

or

2-1/2 inches in diameter.2

Not more than the 

equivalent of 6 scattered 

spots 1/8 inch in diameter in 

a potato 6 ounces in weight 

or

2-1/2 inches in diameter.2

 1 Or correspondingly lesser or greater areas in smaller or larger potatoes.

2 Or correspondingly lesser or greater number of spots in smaller or larger potatoes.
Inspection Instructions for Potatoes for Processing (August 2015)

22

EQUATION OF AREAS

Equation of areas on smaller or larger potatoes is in reference to the following internal 

defects:

• Hollow Heart;

• Hollow Heart with Discoloration; and

• Light Brown Discoloration (Brown Center).

These dimensions are based on area affected, not aggregate area.

Potato is:

Maximum Area allowed

(Diameter)

Processing No. 1

Maximum Area allowed

(Diameter)

Processing No. 2

4 ounces 3/8 inch 5/8 inch

6 ounces 1/2 inch 3/4 inch

7 ounces 5/8 inch 7/8 inch

8 ounces 3/4 inch 1 inch

10 ounces 7/8 inch 1-1/4 inches

14 ounces 1 inch 1-1/2 inches

16 ounces 1-1/8 inches 1-5/8 inches

20 ounces 1-1/4 inches 1-7/8 inches

24 ounces 1-3/8 inches 2 inches

28 ounces 1-1/2 inches 2-1/4 inches

32 ounces 1-5/8 inches 2-1/2 inches

Use color chip POT-CC-1 when determining the effect of “Brown Center” or “Brown 

Core.” Score as damage or serious damage when the affected area is at least this 

shade or darker, and exceeds the specified area. Measure the affected area with a 

diameter gauge (plastic card). The potato is not damaged or seriously damaged until 

the affected area (when the plastic gauge is laid on it) extends beyond the confines of 

the appropriate measured circle.
Inspection Instructions for Potatoes for Processing (August 2015)

23

CUTTING PROCEDURES FOR INTERNAL DEFECTS

DEFECTS NOT ENTIRELY CONFINED TO THE VASCULAR RING

Internal discoloration affecting the vascular ring or occurring between the vascular ring 

and the skin of the potato is scored on a waste basis. Make an exploratory cut on a 

minimum of five potatoes. When scorable defects are found, see Sampling Procedures 

for Internal Defects section.

1. Special consideration is given to Net Necrosis. Although it may be found on the 

side of the potato, it is generally confined to the stem end, and is consequently 

scored on a waste basis. See Figure 1.

2. Nematodes with no external indication are treated as an internal defect and 

scored at the internal defect determination area. Make numerous cuts at 1/8-

inch intervals from stem to blossom end of the potato.

NOTE: If nematodes show both external and internal indications, treat all of it as either 

an internal defect or an external defect depending on which has the greatest amount.

Figure 1

For Net Necrosis, Stem End Browning, Vascular Browning, Internal Black Spot, 

and similar defects clip the stem end of the tuber, prior to using waste method 

below.

In cutting to remove waste caused by Net Necrosis and other similar discoloration at the 

stem end, use a straight cut if the discoloration extends more than 1/2 of the 

circumference around the potato. If the discoloration affects 1/2 or less of the 

circumference around the potato, make the cut on that particular side, using a curved 

cut to reduce the removal of uninjured flesh.
Inspection Instructions for Potatoes for Processing (August 2015)

24

DEFECTS OCCURRING ENTIRELY WITHIN THE VASCULAR RING 

Defects occurring entirely within the vascular ring are determined on a more complex 

basis.

1. For Hollow Heart and Hollow Heart with Discoloration and Light Brown 

Discoloration (Brown Center) make one cut from stem to blossom end parallel to 

the longitudinal axis. Expose the greatest area possible when making this cut. 

See Figure 2.

Figure 2

For Hollow Heart, Hollow Heart with Discoloration and Brown Center, type of 

defects, cut the tuber lengthwise.
Inspection Instructions for Potatoes for Processing (August 2015)

25

2. For Internal Brown Spot and other similar discoloration (heat necrosis). The 

potato is cut at 1/2-inch intervals perpendicular to the longitudinal axis. See 

Figures 3 and 4.

Figure 3

For all other internal discoloration type defects that occur within the vascular 

ring, cut the potato in half-inch slices.

NOTE: Dotted lines indicate location of internal discolored areas.

Figure 4

Slices laid out for examination. Consider only spots on upper side of the slice.
Inspection Instructions for Potatoes for Processing (August 2015)

26

SAMPLING PROCEDURES FOR INTERNAL DEFECTS

• When determining damage and serious damage for grade determination, draw a 

subsample from each “breakdown” or “determination.”

• When determining damage and serious damage for various sizes “breakdown,” 

draw subsamples from each. In most instances, the previous bullet and this 

bullet are combined for determining internal defects for size as well as grade.

• When determining damage and serious damage for the lot as a whole, use a 

composite sample taken from all subsamples at the time of unloading. In the 

event a sample divider is used, intermittent samples taken from the unused 

portion may be utilized.

SAMPLE SIZE

Use the following procedure when cutting for internal defects and bruise free 

determination:

Undivided sample:

• Cut a minimum 10% of each subdivision of grade or size.

• Cut at least 10 pounds if a subdivision is less than 100 pounds.

• Cut entire contents of a subdivision if it is less than 10 pounds.

Divided Sample:

• Cut a minimum 15% of each subdivision of grade or size.

• Cut at least 15 pounds if a subdivision is less than 100 pounds.

• Cut the entire contents if a subdivision is less than 15 pounds.
Inspection Instructions for Potatoes for Processing (August 2015)

27

PINK EYE–PINK ROT

Most processor/grower contracts for potatoes going into storage make reference to 

potatoes affected by symptoms characteristic of Pink Eye-Pink Rot. This is usually 

found in the Definition, Refusal or Tare sections of a contract, all of which emphasizes 

the importance and seriousness of this factor.

Contracts for potatoes going directly into plants or coming out of storage for immediate 

use may not contain any reference to “symptom(s) characteristic of Pink Eye-Pink Rot.” 

These factors would be scored as damage or serious damage, unless progressing to 

the point of soft rot (soft, mushy or leaking condition).

Pink Eye-Pink Rot is extremely difficult to diagnose, even by expert pathologists, so it is 

important that the proper procedures are followed for identification and verification of the 

presence of characteristic symptom(s).

These symptoms include:

• Pink to brown patchy discoloration around the eye, or 

• Discolored tissues around the eyes have turned light brown and have become 

wrinkled and cracked.

Depending on contract specifications, two separate guidelines (free from basis or waste 

basis) must be followed when handling these symptoms(s).

FREE FROM BASIS

When upon casual external examination in the normal process of grading, symptom(s) 

characteristic of Pink Eye-Pink Rot seem to be evident; make an exploratory cut for the 

purpose of verification.

Make exploratory cut(s) at least 1/16 but not more than 1/8 of an inch thick using the 

same procedure as cut(s) made for external defects. When the cut exposes plainly 

visible, irregular sized, tan to pink discolored flesh, score the entire tuber against the 

grade being applied as showing “Symptom(s) characteristic of Pink Eye-Pink Rot” (see 

Figure 5: Free From Basis).
Inspection Instructions for Potatoes for Processing (August 2015)

28

Figure 5: Free From Basis

NOTE: Do not use the underside of the peel or slice in the verification of this disorder.

WASTE BASIS

Scoring Guide

Damage: When causing more than 5% waste.

Serious damage: When causing more than 10% waste.

NOTE: If it has progressed to a soft, mushy or leaking condition, score and report as 

soft rot.
Inspection Instructions for Potatoes for Processing (August 2015)

29

FREE FROM DEFECTS

“Free from defects” are serious in nature. Familiarity with free from defects is very 

important because they are usually classed as cull material on which refusal may be 

based. Free from defects are scored on sight (e.g., any amount is scorable) and 

include the following.

BACTERIAL RING ROT

Bacterial Ring Rot is an important potato disease. It develops rapidly when stored at 64 

to 72 °F and only slightly at 37 °F. Infected potatoes may appear healthy at harvest, but 

can develop characteristic symptoms in storage. Potatoes affected by Bacterial Ring 

Rot are very susceptible to secondary infection especially soft rot organisms. Potatoes 

from diseased plants may show various stages of decay up to complete disintegration. 

Bring suspected tubers to the supervisor’s attention for verification.

Scoring Guide

Always serious damage.

NOTE: If the tuber is affected to the extent that a wet exudate is readily squeezed out, 

or the Bacterial Ring Rot is advanced to the stage of breaking down into a soft and 

mushy condition, score and report as Soft Rot.

BLACKHEART

Blackheart develops in tubers while they are growing or while they are in transit or 

storage. It occurs in all varieties of potatoes from all growing areas. It is important 

because normally no external symptoms are apparent, and the blackened internal 

tissue makes the tubers worthless for processing purposes. The internal symptoms are 

a dark gray to purplish discoloration, which later becomes jet black. The discolored 

areas are usually sharply set off from the healthy tissues. The discoloration is generally 

restricted to the heart of the potato, but may radiate to the exterior. Excessive heat 

and/or the lack of oxygen cause these symptoms. On washed potatoes the external 

appearance of tubers affected by Blackheart may appear duller than tubers not affected.

If discovered on the grading table at the defect determination area during the course of 

normal grading, bring it to the attention of the crew chief or the person(s) performing 

internal grading.

Scoring Guide

Always serious damage.
Inspection Instructions for Potatoes for Processing (August 2015)

30

FREEZING / FREEZING INJURY

Freezing may occur when temperatures drop below 30.9°F. Use the term “frozen” only 

when ice crystals are present. Use the term “freezing injury” when potatoes are not in a 

frozen condition at time of inspection (they may or may not have been frozen but show 

injury).

There are basically three types of freezing injury:

1. Field frost (freeze damage): Usually occurs prior to harvest and appears as 

bluish-gray blotches beneath the skin, generally on one side of the tuber. The 

affected area may also be dry, leathery and severely sunburned from exposure.

2. Freezing necrosis: Causes blackening of vascular elements of the tuber, results 

in a net-like pattern, scattered throughout the tuber within the vascular ring.

3. Blotch necrosis: Usually forms irregular areas of various sizes, ranging from 

opaque blue gray to black.

Report these three types as “Freezing Injury.”

Scoring Guide

Always serious damage.

NOTE: If potatoes become soft and watery after thawing, score and report as “Wet 

Breakdown”. If there is evidence of secondary infection after thawing (e.g., 

disintegrates upon touch, mushy, leaky, moldy, etc.), score and report as “Soft Rot.”

LATE BLIGHT TUBER ROT

Late Blight Tuber Rot is also an important disease of potatoes. During wet, cool 

seasons it may cause serious crop losses in any potato growing area. When not 

complicated by secondary organisms, the decay is reddish-brown or purplish-brown in 

color and spreads irregularly from the surface through the flesh like the diffusion of a 

brown stain. Under storage conditions, the disease is typically a dry rot, forming 

moderately firm, dry, or leathery irregular sunken patches, which under favorable 

conditions, such as high humidity and temperatures, may involve the entire potato. 

These patches frequently have a metallic tinge, especially on the border of healthy 

tissue.

When secondary bacteria invade during storage, the potatoes enter the wet rot stage.
Inspection Instructions for Potatoes for Processing (August 2015)

31

Scoring Guide

Always serious damage: When present in any degree and in a dry rot stage, report as 

“Dry rot” and score against the serious damage tolerance for the grade being applied. 

When in a wet rot stage, report as “Soft rot” and score against the 2% soft rot tolerance.

SOFT ROT / WET BREAKDOWN

All rots, regardless of the name or type, are scored and reported as “Soft Rot” when the 

rot is soft, mushy, or in a leaky condition.

Potatoes that exhibit a wet leaking condition after thawing, score and report as “Wet 

Breakdown.”

Scoring Guide

Always serious damage: Classify all tubers affected by soft rot or wet breakdown as 

unusable material regardless of the area affected.

SOUTHERN BACTERIAL WILT (BROWN ROT)

This disease may attack all varieties of potatoes with varying degrees of severity. It is 

primarily a disease of potato crops in the South Atlantic and Gulf Coast States. For this 

reason, it is often referred to as Southern Bacterial Wilt (SBW). However, the disease 

has been reported as far north as Ohio, Michigan, and Idaho. In the early stages, the 

potatoes, when cut, show the vascular ring is moist and brown with a slight exudation 

from the discolored ring. In many cases, pockets develop in the vascular region, 

containing droplets of bacterial ooze. Frequently, there are no external indications, and 

the presence of the disease may be detected only by cutting the potatoes. An external 

indication of this disease can sometimes be sunken spots at the stem, or gray colored 

spots on the surface. In advanced stages SBW is known as Brown Rot.

Scoring Guide

Always serious damage: Report as “Southern Bacterial Wilt.”

NOTE: If SBW has advanced to the Brown Rot stage (soft and mushy), score and 

report as “Soft Rot.”
Inspection Instructions for Potatoes for Processing (August 2015)

32

WORMS AND INSECTS

Scoring Guide

Always serious damage: When worms or insects are present in a tuber (meaning the 

worms or insects are embedded or burrowed in the potato), the entire tuber is classed 

as unusable material.

NOTE: When worms or insects are not present in a tuber, worm or insect injury are not 

free from defects and injury is determined on a waste basis. When more than 5% waste 

score as damage and when more than 10% waste score as serious damage.

Nematodes are scored differently (see External Defects / Nematodes section).

OPTIONAL TESTS

Tests for specific gravity and fry color are not mandated in the U.S. Standards for 

Grades of Potatoes for Processing; these tests are available when specifically 

requested. Most processing contracts include incentive and/or refusal clauses based 

on specific gravity. 

SPECIFIC GRAVITY

Accurate determination of specific gravity is extremely important to the processing 

industry. It is the means by which the percentage of dry-matter content (solids) of a lot 

is calculated. The percentage of dry-matter has a direct effect on the processing quality 

of the raw product as well as the quality of the finished product. Determine the specific 

gravity in accordance with the following procedure:

SAMPLING FOR SPECIFIC GRAVITY

The potatoes used for such determination will be:

• Taken randomly from a composite sample drawn from containers throughout a 

load; or

• A sample from a bulk load or storage bin(s); or

• From a portion of the divided sample which was initially drawn or submitted for 

grade or contract determinations.

Potatoes utilized in the determination of specific gravity will be clean and taken from the 

general run of the lot (field run) or from specific grade breakdowns depending on 
Inspection Instructions for Potatoes for Processing (August 2015)

33

contract and/or request specification. Use potatoes for specific gravity that are free 

from all rots and obvious free from defects. Whenever possible, do not use potatoes 

that have been cut, either for external or internal defects. Cut potatoes with excessive 

exposed surface may cause a false specific gravity reading. Avoid using the size of 

tubers showing the occurrence of hollow heart in the specific gravity sample.

DETERMINING SPECIFIC GRAVITY

Specific gravity can be determined by two acceptable methods: The Weight in Air and 

Weight in Water Method or the Hydrometer Method.

Weight in Air and Weight in Water Method:

This method consists of weighing a representative sample in air and then weighing the 

same sample when immersed in water. Keep water reasonably clean.

Select from the original sample drawn, three samples of at least 5000 grams (about 11 

pounds each). The specific gravity of the sample is calculated as: [weight in air] 

divided by [weight in air minus weight in water] equals [specific gravity].

Examples:

• If three samples of potatoes weigh 5000 grams each in air and 396, 414, and 388 

grams in water the specific gravity for this lot would be:

5000 grams ____

5000 - 396 grams = 1.0860

5000 grams ____

5000 - 414 grams = 1.0903

5000 grams ____

5000 - 388 grams = 1.0841

 Total 3.2604 ÷ 3 = 1.0868

• If three samples of potatoes weighed 11 pounds each in air and 0.88, 0.91, and 

0.86 pounds in water, the specific gravity for this lot would be: 

11 lbs.

11 lbs. - 0.88 = 1.0870

11 lbs.

11 lbs. - 0.91 = 1.0902

11 lbs.

11 lbs. - 0.86 = 1.0848

 Total 3.2620 ÷ 3 = 1.0873
Inspection Instructions for Potatoes for Processing (August 2015)

34

The specific gravity figure is then corrected for temperature variations as prescribed by 

Table I in the standards. The corrected specific gravity for any lot of potatoes will be the 

average of the 3 corrected readings.

When requested to convert the weight in water of 5000 gram samples used in the 

weight in air versus weight in water method of specific gravity determinations, use the 

conversion to total solids based on Table II in the standards.

NOTE: When using Tables I and II, do not round gravity figures or drop the last digit. 

Both tables are included in these instructions.

Hydrometer Method:

Although this method is seldom used in potatoes for processing, the equipment and 

procedures used are listed below:

Equipment

• 30 gallon drum.

• Scale that weighs in pound graduations.

• Potato hydrometer.

• Basket.

• Steel rod 10-1/2” x1/2.”

Calibration

• Place steel rod in the basket instead of eight pounds of potatoes.

• Attach basket and rod to the hydrometer bulb and lower into drum of water.

• Ensure the hydrometer reads 1.070. If it does not, adjust the chart in the 

hydrometer until the water level of the hydrometer is at 1.070.

From the original sample select, three-eight pound washed samples of uniform and 

average size potatoes for the lot, unless contract specifications state otherwise. The 

sample should be free from defects, particularly hollow heart and relatively free from cut 

surfaces. It may be necessary to cut one potato in the sample to obtain exactly eight 

pounds.
Inspection Instructions for Potatoes for Processing (August 2015)

35

Place one eight pound sample in the wire basket and then:

• Hold the potatoes and the basket in one hand and, with the other, attach the 

hook of the bulb onto the wire loop in the center of the basket.

• Place the potatoes and bulb into the 30-gallon drum of clean water and allow the 

basket containing the potatoes to sink slowly.

• Find the specific gravity by reading the chart in the neck of hydrometer at water 

level.

Repeat this process for the other two samples. Record the pulp temperature of the 

potatoes and the water temperature of each sample. Record all data on the Specific 

Gravity Control Sheet as described in the weight in air-weight in water method. The 

corrected specific gravity of any lot of potatoes will be the average of 3 corrected 

readings.
Inspection Instructions for Potatoes for Processing (August 2015)

36

EXAMPLE OF SPECIFIC GRAVITY NOTESHEET:

SPECIFIC GRAVITY CONTROL SHEET

Grower _______ Date ___________

Field _______ Page of ___________

Storage _______ Certificate No. ___________

Bin __________ Inspector ______________

Scale 

Ticket 

No.

Sample 

Weight 

In 

Water

Sample 

Weight 

In Water

Weight 

Air 

Minus 

Weight 

In Water

Specific 

Gravity

Tuber 

Temp.

Water 

Temp.

Temp. 

Correction

Corrected 

Specified 

Gravity

(1) (2) (3) (4) (5) (6) (7) (8) (9)

NOTE: Above columns are not numbered on the actual control sheet, however, for the 

sake of clarity they are numbered here. Normally, only fill out numbers (1), (2), (3), (6), 

and (7). There are still some plants where you are responsible for computing and 

entering all vital statistics on the control sheet.

NOTE: When requested to convert the weight in water samples used in the weight in 

air versus weight in water method of specific gravity determinations, use Table II to 

covert to total solids.
Inspection Instructions for Potatoes for Processing (August 2015)

37

TABLE I – CORRECTION FACTORS FOR SPECIFIC GRAVITY OF POTATOES1

(Corrected to zero at 50 °F potato temperature and 50 °F water temperature)

Tuber temp

(°F)

Water temperature (°F)

38 40 45 50 55 60 65 70 75 80

38........... -.0021 -.0020 -.0018 -.0018 -.0020 -.0023 -.0029 -.0038 -.0047 -.0056

40........... -.0017 -.0016 -.0014 -.0014 -.0016 -.0019 -.0025 -.0034 -.0043 -.0052

45........... -.0009 -.0008 -.0006 -.0006 -.0008 -.0011 -.0017 -.0026 -.0035 -.0044

50........... -.0003 -.0002 0000 0000 -.0002 -.0005 -.0011 -.0020 -.0029 -.0038

55........... +.0001 +.0002 +.0004 +.0004 +.0002 -.0001 -.0007 -.0016 -.0025 -.0034

60........... +.0004 +.0005 +.0007 +.0007 +.0005 +.0002 -.0004 -.0013 -.0022 -.0031

65........... +.0005 +.0006 +.0008 +.0008 +.0006 +.0003 -.0003 -.0012 -.0021 -.0030

70........... +.0006 +.0007 +.0009 +.0009 +.0007 +.0004 -.0002 -.0011 -.0020 -.0029

75........... +.0007 +.0008 +.0010 +.0010 +.0008 +.0005 -.0001 -.0010 -.0019 -.0028

80........... +.0008 +.0009 +.0011 +.0011 +.0009 +.0006 0000 -.0009 -.0018 -.0027

85........... +.0009 +.0010 +.0012 +.0012 +.0010 +.0007 +.0001 -.0008 -.0017 -.0026

90........... +.0010 +.0011 +.0013 +.0013 +.0011 +.0008 +.0002 -.0007 -.0016 -.0025

95........... +.0011 +.0012 +.0014 +.0014 +.0012 +.0009 +.0003 -.0006 -.0015 -.0024

100......... +.0012 +.0013 +.0015 +.0015 +.0013 +.0010 +.0004 -.0005 -.0014 -.0023

Whenever a temperature falls between two sets of numbers use the following rounding procedures: When the number is 

less than 2.5 round down to the next lowest temperature, for example 52.4o rounds down to 50o. Round up to the next 

highest temperature when the number is 2.5 or more; for example 52.5o rounds up to 55o

.

 1 To apply the correction factor, change the actual specific gravity reading by adding or subtracting the appropriate factor according to the plus or minus sign.
Inspection Instructions for Potatoes for Processing (August 2015)

38

TABLE II – SPECIFIC GRAVITY CONVERSION CHART

For 5,000 grams of potatoes

Water

Weight

Specific 

Gravity

Total 

Solids 

Water 

Weight

Specific 

Gravity

Total 

Solids

Water 

Weight

Specific 

Gravity

Total 

Solids

300 1.0638 17.2 358 1.0771 20.1 400 1.0870 22.2

310 1.0661 17.7 360 1.0776 20.2 402 1.0874 22.3

320 1.0684 18.2 362 1.0780 20.3 404 1.0879 22.4

322 1.0688 18.3 364 1.0785 20.4 406 1.0884 22.5

324 1.0693 18.4 366 1.0790 20.5 408 1.0888 22.6

326 1.0697 18.5 368 1.0794 20.6 410 1.0893 22.7

328 1.0702 18.6 370 1.0799 20.7 412 1.0898 22.8

330 1.0707 18.7 372 1.0804 20.8 414 1.0903 22.9

332 1.0711 18.8 374 1.0808 20.9 416 1.0908 23.0

334 1.0716 18.9 376 1.0813 21.0 418 1.0912 23.1

336 1.0720 19.0 378 1.0818 21.1 420 1.0917 23.2

338 1.0725 19.1 380 1.0822 21.2 422 1.0922 23.4

340 1.0730 19.2 382 1.0827 21.3 424 1.0926 23.5

342 1.0734 19.3 384 1.0832 21.4 426 1.0931 23.6

344 1.0739 19.4 386 1.0836 21.5 428 1.0936 23.7

346 1.0743 19.5 388 1.0841 21.6 430 1.0941 23.7

348 1.0748 19.6 390 1.0846 21.7 432 1.0946 23.9

350 1.0753 19.7 392 1.0851 21.8 440 1.0965 24.4

352 1.0757 19.8 394 1.0855 21.9 450 1.0989 24.9

354 1.0762 19.9 396 1.0860 22.0 460 1.1013 25.4

356 1.0766 20.0 398 1.0865 22.1 470 1.1040 26.0

Whenever the recorded water weight for an individual reading falls between two sets of numbers as indicated above, use 

the next higher reading. Example: For a water weight reading of “371,” use the “372” specific gravity reading.
Inspection Instructions for Potatoes for Processing (August 2015)

39

GLUCOSE CONTENT

In the manufacture of French fries and related products, the sugar content of the 

potatoes is closely related to the color produced. The processor’s quality control staff 

usually performs the determination of glucose content (also called testing for reducing 

sugar). However, there are times the Inspection Service is asked to run the glucose 

test. When requested, use the following procedures:

• Select 15 potatoes at random from the graded sample or from the non-graded 

portion of the divided sample, provided the tubers are free from defects.

• Cut each potato lengthwise, from stem to eye end.

• Place the glucose test tape on the cut surface on the long axis in the middle of 

the potato. Ensure that the piece of tape used is sufficiently long so that the 

portion of the tape touched by fingers is not placed on the cut surface.

• Replace the other half of potato to moisten tape. Separate the halves, remove 

tape, and, after 1 minute, compare the color of the tape with the color chart on 

the tape container.

• Typically, contract specifications state that if more than 5% of the test tape is 

darker color than permitted (when compared to the tape container chart) the 

glucose content is considered excessive. A chart contained in these instructions 

will serve as a guide in determining if more than 5% of the area of the test tape is 

darker color than permitted. Do not average test tape readings.
Inspection Instructions for Potatoes for Processing (August 2015)

40

NOTE: The two solid areas in each rectangle represent the maximum amount of an 

equivalent strip of test tape, when applied to a potato of the same size, which is 

permitted to indicate the presence of more than one tenth of 1% glucose. Each solid 

area represents 2-1/2% of the total length of the tape.

Most contracts specify or the processor’s quality control guidelines do not permit 

glucose content of more than one-tenth of one percent on more than 5% of the surface.
Inspection Instructions for Potatoes for Processing (August 2015)

41

FRY COLOR

Frying quality of potatoes is very important in potatoes for processing. Processors may 

refuse lots with undesirable frying characteristics, which do not meet contract 

specifications. 

When sugar ends, high sugar and/or fry color determination is requested. Use the 

following procedures for determining fry color (unless otherwise specified by request or 

contract specification):

• Randomly select a minimum of 20 potatoes free of external defects occurring on 

the ends.

• Cut each potato lengthwise in 1/2 inch (12.7 mm) square strips (unless otherwise 

specified by request or contract specification). Ensure stem end is toward the 

blade and take care not to damage the ends of the strips. Obtain one strip as 

near the center as possible from each potato that is free from internal defects.

• Place the strips in the basket.

• Place basket containing strips in fryer after the temperature of the oil is at least 

350 °F, but not over 375 °F.

• Fry for 3 minutes at 350 °F or 2-1/2 minutes at 375 °F. It is critical to check 

temperature of oil with a thermometer.

• Remove strips from oil and place on white surface.

• Determine fry color within 2 minutes of removal from oil.

NOTE: Change oil daily when there is heavy usage. Change when it is obviously dirty; 

contains a moderate amount of suspended or sunken particles; noticeably smoking; or 

has an apparent off-odor. It is advisable to use the same type and quality of oil as being 

used in processing plant frying operations.

The fried strips may be compared to the following:

• Munsell Color Standards for Frozen French Fried Potatoes and reported 

according to contract specifications.

• USDA approved colorimeter, Agtron H-30A or H-300A, or any other approved 

electronic colorimeter, that has been properly calibrated and standardized. 

Contract specifications must specify one or more of the color designations with 

corresponding colorimeter indices.
Inspection Instructions for Potatoes for Processing (August 2015)

42

After sample fry strips are prepared and test fried, and in absence of a colorimeter, 

determine fry color by comparing the darkest side of each sample fry strip with the 

Munsell Color Chart for French Fried Potatoes. In determining the 0, 1, 2, 3, or 4 color 

of the fry strip, a fry strip may show a darker variation than the numbered illustrations. 

For example, a fry strip may have a color that falls between a number 2 and a number 

3, but is still a number 2 until it reaches the color equal to a number 3.

Report fry color using the following definitions unless otherwise specified.

Sugar End: A sample fry strip which when prepared and test fried has a predominant 

color of number 3 or darker when compared with the USDA Munsell Color Chart for 

French Fried Potatoes on any two sides extending 1/2 inch or more from the end of the 

sample fry strip.

High Sugar: A sample fry strip which when prepared and test fried has a predominant 

color of number 3 or darker when compared with the “USDA Munsell Color Chart for 

French Fried Potatoes” on any two sides of the fry strip affecting 1/2 the length or more 

from the end of the sample fry strip.

Predominant Color: An area with more than one color which is scored based on the 

predominant color. For example, if 1/3 of the area is a number 3 color and 2/3 of the 

area is a number 1 color, score the area as a number 1.

BRUISE FREE

“Bruise free” is the complete absence of any discoloration due to impact in the handling 

process, no matter how slight. Contracts and/or requests may be written to vary from 

definition for various reasons.

DETERMINING BRUISE FREE

Bruise free can only be determined after the complete removal of the skin. If the 

amount of peel removed is not specified on a grower-processor contract, for 

consistency and uniformity, use a range of 10% to 15% peel loss. If peel loss outside 

this range occurs, adjust the equipment to correct the problem. The sample is still valid 

regardless of the percentage of peel loss.

Unless otherwise directed by the applicant or contract specifications, use the following 

definitions to distinguish “old bruises” from “fresh bruises” in the bruise free 

determination when potatoes are coming out of storage:

Fresh Bruises: Typically a shade of pink or a bright shiny gray to jet black color and 

show no sign of dry or dry starchy flesh.
Inspection Instructions for Potatoes for Processing (August 2015)

43

Old Bruises: Typically dull gray, light brown or other colors, which may show a dry or 

dry starchy appearance in the flesh. Also, old bruises may show a separation of the 

flesh or some corkiness of the flesh.

NOTE: Applications of the above definitions are characteristic of bruises within a 48 

hour period from initial movement of potatoes from storage.

REMINDERS ABOUT BRUISE FREE

In all cases, bruise free is a very important part of processor/grower contracts. Keep in 

mind the following points concerning bruise free. For lighting requirements refer to the 

Inspection Facilities and Equipment section.

• Score all bruising when coming from the field.

• Unless otherwise specified by contract, hold samples of potatoes going into 

storage at least 48 hours before making the bruise free determination.

• Score only old bruises when coming out of storage, unless otherwise specified by 

contract.

• Contracts generally specify size of bruise allowed.

• Do not score bruises that were clearly a result of grading machinery.

• Score Internal Black Spot, which normally occurs when potatoes experience a 

slight jolt or bruise during handling, as bruises.

• Unless otherwise specified, use weight as the basis of calculating percentages1.

• Even though plant personnel regulate the caustic peeler solution, bring to their 

attention if tubers are inadequately peeled or if excessively peeled.

• If two sets of electronic eyes are used to count bruise free tubers and one set of 

eyes records a higher count of “unbruised” tubers, use the higher count as the 

official count.

• When electronic eyes do not count properly, bring to the immediate attention of 

plant personnel.

• Most bruise free grading lines are in the proximity of the peeler; if this is the case, 

make personal safety a priority. Proper use of personal protective equipment 

 1 Determine the percentage of bruise free by dividing the weight of the potatoes found to be “bruise free” by the total weight of all 

peeled potatoes used in the bruise free sample.
Inspection Instructions for Potatoes for Processing (August 2015)

44

such as goggles and splash aprons, may be required if you are exposed to the 

chemicals used in a caustic peeler.